This change removes the need for poll_events on a Window, per issue #31.

It removes the stream-based event handling in favor of a Window-local
map of pointers to registered callback closures. These closures are
owned by the raw C window and are released when the window is destroyed.
Remove extra whitespace.
Merge pull request #32 from larsbergstrom/remove-poll-events

This change removes the need for poll_events on a Window, per issue #31.